Re: [Qemu-devel] [PATCH] kvm: Set default accelerator to "kvm" if the host supports it

[Aurelien Jarno <aurelien@aurel32.net>]

On Mon, Oct 01, 2012 at 11:20:41AM -0500, Anthony Liguori wrote:
> Jan Kiszka <jan.kiszka@siemens.com> writes:
> 
> > If we built a target for a host that supports KVM in principle, set the
> > default accelerator to KVM as well. This also means the start of QEMU
> > will fail to start if KVM support turns out to be unavailable at
> > runtime.
> >
> > Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> > ---
> >  kvm-all.c  |    1 +
> >  kvm-stub.c |    1 +
> >  kvm.h      |    1 +
> >  vl.c       |    4 ++--
> >  4 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/kvm-all.c b/kvm-all.c
> > index 92a7137..4d5f86c 100644
> > --- a/kvm-all.c
> > +++ b/kvm-all.c
> > @@ -103,6 +103,7 @@ struct KVMState
> >  #endif
> >  };
> >  
> > +bool kvm_configured = true;
> >  KVMState *kvm_state;
> >  bool kvm_kernel_irqchip;
> >  bool kvm_async_interrupts_allowed;
> > diff --git a/kvm-stub.c b/kvm-stub.c
> > index 3c52eb5..86a6451 100644
> > --- a/kvm-stub.c
> > +++ b/kvm-stub.c
> > @@ -17,6 +17,7 @@
> >  #include "gdbstub.h"
> >  #include "kvm.h"
> >  
> > +bool kvm_configured;
> >  KVMState *kvm_state;
> >  bool kvm_kernel_irqchip;
> >  bool kvm_async_interrupts_allowed;
> > diff --git a/kvm.h b/kvm.h
> > index dea2998..9936e5f 100644
> > --- a/kvm.h
> > +++ b/kvm.h
> > @@ -22,6 +22,7 @@
> >  #include <linux/kvm.h>
> >  #endif
> >  
> > +extern bool kvm_configured;
> >  extern int kvm_allowed;
> >  extern bool kvm_kernel_irqchip;
> >  extern bool kvm_async_interrupts_allowed;
> > diff --git a/vl.c b/vl.c
> > index 8d305ca..f557bd1 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -2215,8 +2215,8 @@ static int configure_accelerator(void)
> >      }
> >  
> >      if (p == NULL) {
> > -        /* Use the default "accelerator", tcg */
> > -        p = "tcg";
> > +        /* The default accelerator depends on the availability of KVM. */
> > +        p = kvm_configured ? "kvm" : "tcg";
> >      }
> 
> How about making this an arch_init() function call and then using a #if
> defined(KVM_CONFIG) in arch_init.c?
> 
> I hate to introduce another global variable if we can avoid it...
> 
> Otherwise:
> 
> Acked-by: Anthony Liguori <aliguori@us.ibm.com>
> 
> Blue/Aurelien, any objections?
> 

I am not sure this way of doing is really distribution friendly, where
the QEMU package is built for a large variety of hosts, some of them
maybe without KVM support.

I am all for enabling KVM by default, but I think it should fall back to
TCG if it is not enabled (probably with a warning so that the user is
aware of that). On the other hand, if the user explicitly enables KVM
support with -enable-kvm or with accel=kvm, it should fail to start. In 
other words, a bit like the configure script options, by default we 
use auto-detection, but if an option is explicitly enabled, it fails if
it can't be enabled.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net